1. Field of the Invention
The present invention relates to a variable magnification lens system with a constant object-image distance for use in copying machines or the like, and more particularly, to a lens system capable of continuously varying the magnification which includes equal magnification while keeping the distance between the object and image planes constant.
2. Description of the Prior Art
Recently, an optical system capable of continuously varying the magnification of the image was introduced for in copying machines and machines for making up printing originals or the like. When a lens with a fixed focal length is used in such an optical system, it is necessary to alter the conjugate distance by shifting of the lens. The conjugate distance is altered by shifting a mirror provided in the optical path, for example. However, the optical system mentioned above has the drawback of necessitating a complicated shift mechanism for the mirror.
For eliminating the above drawback, it was proposed to use a variable magnification lens system with a constant object-image distance, for example, in Japanese Laid Open Patent Application No. 57-67909. This patent application discloses a lens system having a first lens unit with negative refractive power, a second lens unit with positive refracting power, a third lens unit with positive refracting power and a fourth lens unit with negative refractive power, wherein the second and third lens units are movable along the optical axis symmetrically with the central plane of the lens system while fixing the first and fourth lens units. The system provides a magnification range from -1.58 to -0.63.
The lens system disclosed in Japanese Patent Application No. 57-67909, however, can not have a wider range of magnifications than the above-mentioned range because the coma aberration increases in the off-axial region if the range is widened, and it further necessitates a long object-image distance for obtaining the same optical performance as that of a lens of a fixed focal length. In addition, aberrations of this lens system increases as the magnification varies from the equal magnification.